That is a plate mounted on a part confirming that required modifications have been carried out. "Mods" are formal directives to perform upgrades or design revisions either on the factory floor or in the field.
If you are asking specifically what those numbered mods entail for the Halifax then someone with access to the modification leaflets, which outline in detail exactly what is to be done, or a mod index, which is just a list of the mod titles, would have to respond.